Rewritable/erasable media, such as magnetic disks and tapes and magneto-optical disks, are relatively inexpensive technologies for storing computer data. However, the ability to erase and rewrite data stored on the media renders such technologies deficient when it is desired to store permanent information which should not be altered, such as legal documents, financial and insurance records, and the like. Write-protect mechanisms can be used to prevent accidental erasure or alteration of data but the mechanisms are easily reversed and are not adequate to prevent intentional erasure or alteration.
Write-once, read-many (WORM) media, such as ablative and bubble/pit optical disks, offer permanence for the data recorded thereon. However, WORM media is considerably more expensive than rewritable media and cannot also be used to record non-permanent data.